A reference message of that kind for producing basic cycles in the context of time-controlled communication on a bus system is used, for example, in the case of FlexRay or also in the case of TTCAN. Such bus systems are necessary, since the networking of control units, sensors and actuators has increased dramatically in recent years not only in modern motor vehicle manufacturing and in engineering, especially in the machine tool sector, but also in automation technology. It is possible in these cases to obtain synergetic effects by distributing functions among several control units. Communication between various stations of such distributed systems is increasingly taking place, therefore, via a bus system or via a plurality of coupled bus systems. The communications traffic on the bus systems, access and receiving mechanisms, and error handling are governed by a protocol. One protocol, which is established, for example, in the automotive sector, is the CAN (controller area network) protocol. This is an event-triggered protocol, that is to say, protocol activities such as transmission of a message are initiated by events that originate outside the communications system, that is, outside the bus system. The bus system involved in this case is an event-triggered bus system that may, in particular, also be triggered by external events. As a result, the CAN protocol is very flexible and thus it is possible for further stations and messages to be added without any problem.
An alternative approach to event-triggered, spontaneous communication is the purely time-triggered approach. All communication activities on the bus are strictly periodic. Protocol activities such as the transmission of a message are triggered only by the passage of a time applicable to the entire bus system. Access to the medium is based on the allocation of time ranges in which a transmitting station has an exclusive transmission right. The protocol is comparatively inflexible, and adding new nodes is possible only if the corresponding time ranges were left free beforehand. This circumstance forces the order of the messages to be set before operation is started. In this case, therefore, a schedule is drawn up which has to meet the requirements of the messages with regard to repetition-rate, redundancy, deadlines and so on. At the same time, the positioning of the messages within the transmission periods must be coordinated with the applications producing the contents of the messages so that the latencies between the application and the instant of transmission are kept to a minimum. If that coordination is not carried out, the advantage of time-triggered transmission—minimal latency jitters when the messages are being sent over the bus—would be destroyed. In the case of the purely time-triggered approach, therefore, high demands are made on the planning pool.
The approach using a time-triggered controller area network, the so-called TTCAN, that is presented in the 2003 ISO 11898-4 Standard-Draft (ISO/TC22/SC3) satisfies the requirements outlined above for time-triggered communication and satisfies the requirements for a certain degree of flexibility. The TTCAN fulfills those requirements by the structuring of communication rounds, so-called basic cycles, by sending a reference message using the timer or the so-called time master. Those basic cycles are then divided in turn into so-called exclusive time windows for periodic messages of specific communications stations and into so-called arbitrating time windows for spontaneous messages of a plurality of communications stations.
The following remarks are therefore based on the synchronization not only of at least two TTCAN bus systems or networks but also of other, comparable bus systems already mentioned, for example FlexRay, the intention also being to ensure synchronization of mixed, that is to say, different, bus systems.
In such systems, the communication round is determined, as already mentioned, by a basic cycle, that is, by a timer or a first station that repeatedly transmits a reference message over the bus system at least one specifiable time interval, the reference message being triggered by time trigger information when time information reaches a time mark associated with the trigger information. The time information in the TTCAN is, for example, the cycle time, which is defined by the local time, that is, the local clock of the timer or time master, and by the reference message. If a given time mark is reached by that time information, that is, the cycle time of the TTCAN, then whenever that time mark is reached a trigger will be activated in order to start the respective reference message. In that manner, the time master in the bus system specifies the time for the bus system in accordance with the basic cycle. If a shifting of such a basic cycle is to take place, the TTCAN protocol, for example, offers the possibility of shifting the communication by setting a bit in such a reference message. Such shifting is particularly necessary in order, for example, to synchronize the TTCAN bus with the phase of a time base, especially an external time base, for example if the Event-Synchronized-Time-Triggered-Communication-Option is used in the TTCAN.
That is to say, time-triggered networks or bus systems like the TTCAN just mentioned or also FlexRay offer the possibility in most cases of synchronizing the communication with the phase of an external time base, such as, for example, also that of a further time-triggered network or bus system. The commonest method hitherto, in which, however, no mixed synchronization, that is to say synchronization of various different bus systems, is carried out, is a method that requires several actions of the host controller. Firstly, the host has to communicate to all nodes or stations, by setting corresponding provided bit/bit fields in a reference message or by sending a defined message, that after completion of the communication cycle a time interval, a so-called time gap, will be inserted. The host CPU then has to wait until the time gap has begun. The next reference message is then started in synchronized manner by activating the time trigger.
A disadvantage with such a method, however, is that intervention of the host controller or processor always takes place, and therefore, in the case of synchronizing a plurality of networks or bus systems, a high load is placed on at least one station processor and a certain dependence upon software latencies arises.